This invention relates in general to multiphase inverter systems for AC generators which incorporate static electrical power conversion elements and, more particularly, to a three-phase inverter with a composite output transformer which provides an AC output voltage which is regulated and independent of a DC input voltage, over a predetermined range.
The invention is particularly applicable to aviation electrical power generation requirements which rely on state of the art electronic features, such as variable speed constant frequency (VSCF) electrical power generators with DC-link control, which are needed aboard present and future military aircraft and for multi-engine commercial aircraft as well. It will be appreciated that the invention has broader applications such as wherever it is needed to provide output power voltage regulation from a static inverter without mandating the controlling of the DC input voltage to the inverter.
Each VSCF generator unit aboard an aircraft includes a three-phase power inverter paired with a propulsion engine in order that each engine can be used to help fulfill the requirement of system redundancy to enhance operational safety. This ensures that the entire electrical power output needs for the aircraft will be met in order to keep the aircraft operational. The VSCF system according to U.S. Pat. No. 4,554,501, entitled "Variable Speed Constant Frequency Power System With Auxiliary DC Output," is assigned to the same assignee as the present invention, and it is hereby incorporated by reference into the present application. Another variable speed generator is of the type described in U.S. Pat. No. 4,851,723 entitled "Coolant Pump System for Variable Speed Generators" which also is assigned to the present assignee, and it also provides a proper environment for the presently intended multi-phase inverter system.
The three-phase inverters that are used with DC-link frequency conversion for aircraft power generation systems, operating at 400 Hertz and 20 to 50 Kilovolt-amps (KVA), are a subset of static inverters employing semiconductor switching devices which are used for many other applications. These include industrial drive, power conditioning, and the generation of controllable leading or lagging reactive current. In each of these applications, there is a common problem which is difficult to solve. It is the elimination and/or reduction to filterable minimum values of troublesome AC harmonic frequencies which are whole number multiples of the fundamental frequency in Hertz. One approach to solving the problem of harmonic neutralization is according to U.S. Pat. No. 4,870,557, entitled "Simplified Quasi-Harmonic Neutralized High Power Inverters," which has been assigned to the same assignee as the present invention. The above assigned patent is hereby incorporated by reference into the present application and will be hereinafter referenced as the first incorporated U.S. patent. This first incorporated U.S. patent takes advantage of the double fed twenty-four pulse inverter comprised of a standard group of four six-pulse inverters connected to reduce the complexity of and to half the number of normally required phase shifting transformers with open primary winding neutrals. Furthermore, the secondary winding of at least one of the phase shifting transformers is a delta or a wye winding connected to a neutral line in order to simplify the quasi-harmonic neutralization of the output.
Another timely approach directed toward solving the problem of harmonic neutralization is with the implementation of twelve-pulse and twenty-four pulse inverters assembled from pairs of six-pulse inverters with strategic placement of time-paired notching sets for the reversal of the pole voltage. This approach places each notch with a timing duration within a limited band near to the leading and trailing portions of a half-wave periodicity, and it significantly decreases the original phase displacement of the pole voltages, while offering the simplicity of a single output coupling transformer. This is the subject of pending U.S. No. 07/612,647, filed Nov. 13, 1991 entitled "Static Inverter" assigned to the same assignee as the present invention, and it is hereby incorporated by reference into the present application and will be hereinafter referenced as the first incorporated U.S. application. The clear benefit to be derived in notching the switching output of the inverters, within this leading and trailing portion timing range, is the affect it has on some of the most significant harmonics. This viewpoint is for some of the practical applications of the quasi-harmonic neutralized output voltage waveform in which some harmonics are reduced and/or eliminated; i.e., the fifth and seventh harmonics are eliminated and the eleventh and thirteenth are significantly reduced.
Another timely approach directed toward solving the problem of harmonic neutralization is with the implementation of a six-pole inverter apparatus assembled from a pair of six-pulse bridge inverter circuits which provide three-phase pole output voltages which are switched or notched in an optimized variable placement with respect to preselected phase positions and variable pulse width or duration. This is the subject of copending U.S. patent application Ser. No. 07/679,484 entitled "Voltage and Harmonic Control of a Multi-Pole Inverter" assigned to the same assignee as the present invention, and it is hereby incorporated by reference into the present application and will be hereinafter referenced as the second incorporated U.S. application for patent.
The clear benefit to be derived from notching the switching output of the inverters of the six-pole static inverter is the reduced weight realized by the optimized configuration. This configuration incorporates a pair of neutralizing transformers connected, for example, in a wye with a delta paired configuration in order to reduce the harmonic content of the output voltage of the inverter circuit to a filterable minimum of any harmonic components remaining after cancelling all harmonics of the fundamental output voltage, except for the harmonic voltage components with a frequency immediately next adjacent to a whole number multiple of twelve times the fundamental frequency.
One of the principal practical considerations in the above realization of six-pole static inverter harmonic neutralization of the type described, is in contrast that a twelve-pole static inverter provides further flexibility and a broader zone of control for the AC output voltage. Albeit, there is an inherent weight penalty for harmonically neutralizing components over a predetermined range of voltage control of the fundamental frequency component independent of the DC source voltage. It is anticipated from the invention that a twelve-pole static inverter of a presently known design can be modified to improve harmonic reduction of low-order harmonic components that are troublesome; by way of retrofitting with the present invention, an improved weight reduction is achieved with the need for electrical filtering components of only reduced size and functional limit requirements.
Another problem related to the above concerns is that the low-order harmonic component neutralization with the twelve-pole inverter circuit should eliminate all of the harmonic components below twenty-three times the fundamental frequency.
The solution should further concern the provision that all odd-harmonic components immediately next adjacent to whole number multiples of twenty-four times the fundamental frequency are rendered inconsequential and thereby reduce filtering requirements, over the entire range of control.